Actuator, optical scanner and image forming device

ABSTRACT

An actuator includes a movable plate having a plate shape, a pair of axial parts that is elastically deformable and supporting the movable plate rotatable, and a tension adjuster adjusting tension on an axial direction of the pair of the axial parts and including a torsional axis that is formed jointly or integrally with one of the axial parts and disposed orthogonally to the axial direction of the axial parts, and a drive source that torsionally deforms the torsional axis, wherein the torsional axis is torsionally deformed through the action of the drive source and a spring constant of the pair of the axial parts is adjusted by adjusting the tension on the pair of the axial parts.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an actuator, an optical scanner and animage forming device.

2. Related Art

An optical scanner is embedded in a laser printer or the like. Theoptical scanner is used for drawing an image through optical scanning.JP-A-2004-191953 is an example of related art. The example discloses anoptical scanner that has an actuator using a torsional vibratingelement. The actuator (optical scanner) according to the example has atorsional vibrating element of a single-degree-of-freedom vibrationsystem. More specifically the actuator has a movable plate (a reflectingmirror part) which is supported by a support part (a fixing frame)through an axial part (a spring part) such that the movable plate isrotatable with respect to the support part.

The movable plate rotates when the axial part is distorted and deformed.Light is reflected at the light reflecting part and scanning isperformed. In this way, an image can be drawn through the opticalscanning.

In the actuator according to the example, weight of the movable plateand a spring constant of the axial part are fixed so that the actuatorcan only be driven at a given resonance frequency once it is fabricated.This means if the weight of the movable plate and the spring constant ofthe axial part become different from a design value of the actuator, theresonance frequency also becomes different from a desired value. Forthis reason, it is difficult to obtain an actuator that has a desiredrotating-motion characteristic.

SUMMARY

An advantage of the present invention is to provide an actuator in whicha resonance frequency of a movable plate can be easily and accuratelyadjusted even after it is manufactured and which can exercise a desiredvibration characteristic. Another advantage of the invention is toprovide an optical scanner and an image forming device thereof.

An actuator according to a first aspect of the invention includes amovable plate having a plate shape, a pair of axial parts that iselastically deformable and supporting the movable plate rotatable, and atension adjuster adjusting tension on an axial direction of the pair ofthe axial parts and including a torsional axis that is formed jointly orintegrally with one of the axial parts and disposed orthogonally to theaxial direction of the axial parts, and a drive source that torsionallydeforms the torsional axis, wherein the torsional axis is torsionallydeformed through operation of the drive source and a spring constant ofthe pair of the axial parts is adjusted by adjusting the tension on thepair of the axial parts. In this way, it is possible to provide anactuator in which a resonance frequency of a movable plate can be easilyand accurately adjusted even after it is manufactured and which canexercise a desired vibration characteristic.

It is preferable that the tension adjuster has a pair of the torsionalaxes corresponding to the pair of the axial parts and a pair of thedrive sources corresponding to the pair of the torsional axes. In thisway it is possible to adjust the tension on the pair of the axial parts(or the spring constant of the pair of the axial parts) in a wide range.

It is preferable that a torsional center of the torsional axis bedisposed at a distance from a joint part of the torsional axis with theaxial part in a thickness direction of the movable plate. It is alsopreferable that both ends of the torsional axis in its axial directionbe fixed. In this way it is possible to deform the torsional axistorsionally with a relatively small force.

It is preferable that the torsional axis be coupled with the axial partat a center part of the torsional axis in the axial direction. In thisway it is possible to adjust the tension on the pair of the axial partsin a wider range with a smaller force. It is also preferable that thedrive source torsionally deform the torsional axis toward a direction inwhich the tension on the pair of the axial parts is increased. In thisway it is possible to prevent the pair of the axial parts from beingwarped and it is also possible to adjust the tension on the pair of theaxial parts while securing a stable ration of the movable plate.

Moreover, it is preferable that the drive source further include apiezoelectric element contracting and elongating in a directionorthogonal to the axial direction of the axial part and to an axialdirection of the torsional axis, a piezoelectric element joint partjointed with an end of the piezoelectric element in a contraction andelongation direction of the piezoelectric element, and a coupling partcoupling the piezoelectric element joint part with the torsional axis,wherein the piezoelectric element joint part is displaced throughcontraction and elongation of the piezoelectric element, and thetorsional axis is torsionally deformed through the displacement of thecoupling part caused by the contraction and elongation of thepiezoelectric element. In this way it is possible to deform thetorsional axis torsionally with a relatively simple mechanism.

It is preferable that the piezoelectric joint part be disposed in anopposite side to the movable plate with respect to the torsional axis.In this way it is possible to increase design freedom of the actuator.It is also preferable that the coupling part be coupled with a centerpart of the torsional axis in the axial direction of the torsional axis.In this way it is possible to deform the torsional axis torsionally witha small force.

It is preferable that the coupling part be disposed along an axial lineof the axial part when it is viewed from a plane of the movable plate.In this way, the force generated through the contraction and elongationof the piezoelectric element can be efficiently transmitted to thetorsional axis. It is also preferable that the piezoelectric elementhave a structure in which piezoelectric layers and electrode layers arealternately deposited on top of each other. By adopting suchpiezoelectric element, a driving voltage can be lowered and the amountof the displacement can be increased.

It is preferable that the actuator further include a driver rotating themovable plate where electricity is turned on, wherein the tensionadjuster adjusts the tension on the pair of the axial parts such that afrequency of a voltage that is applied to the driver becomes same as aresonance frequency of the movable plate. In this way, it is possible torotate the movable plate with large amplitude as well as lowering thedriving voltage level. Furthermore, it is preferable that the actuatorfurther include a light reflecting part having a light reflectionproperty and disposed on one face of the movable plate. In this way, theactuator can be applied to various optical devices such as an opticalscanner, an optical switch and an optical attenuator.

In this case, it is preferable that the drive source further include apiezoelectric element contracting and elongating in a directionorthogonal to the axial direction of the axial part and to an axialdirection of the torsional axis, a piezoelectric element joint partjointed with an end of the piezoelectric element in a contraction andelongation direction of the piezoelectric element, and a coupling partcoupling the piezoelectric element joint part with the torsional axis,wherein the piezoelectric element is disposed in an opposite side to thelight reflecting part with respect to the movable plate. In this way itis possible to increase design freedom of the actuator.

An optical scanner according to a second aspect of the inventionincludes a movable plate having a plate shape and having a lightreflecting part that has a light reflecting property and is disposed ona plate face of the movable plate; a pair of axial parts that iselastically deformable and supporting the movable plate rotatable; and atension adjuster adjusting tension on an axial direction of the pair ofthe axial parts and including a torsional axis that is formed jointly orintegrally with one of the axial parts and disposed orthogonally to theaxial direction of the axial parts, and a drive source that torsionallydeforms the torsional axis, wherein the torsional axis is torsionallydeformed through operation of the drive source and a spring constant ofthe pair of the axial parts is adjusted by adjusting the tension on thepair of the axial parts. In this way, it is possible to provide anoptical scanner in which a resonance frequency of a movable plate can beeasily and accurately adjusted even after it is manufactured and whichcan exercise a desired vibration characteristic.

An image forming device according to a third aspect of the inventionincludes an optical scanner that has a movable plate having a plateshape and having a light reflecting part that has a light reflectingproperty and is disposed on a plate face of the movable plate, a pair ofaxial parts that is elastically deformable and supporting the movableplate rotatable, and a tension adjuster adjusting tension on an axialdirection of the pair of the axial parts and including a torsional axisthat is formed jointly or integrally with one of the axial parts anddisposed orthogonally to the axial direction of the axial parts, and adrive source that torsionally deforms the torsional axis, wherein thetorsional axis is torsionally deformed through the action of the drivesource and a spring constant of the pair of the axial parts is adjustedby adjusting the tension on the pair of the axial parts. In this way itis possible to provide an image forming device that can exert a fineimage drawing characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a schematic plan view of an actuator according to anembodiment of the invention.

FIG. 2 is a sectional view of the actuator along the line A-A in FIG. 1.

FIG. 3 is a schematic sectional view for describing movements of atension adjuster which is disposed in the actuator shown in FIG. 1.

FIG. 4 is a schematic sectional view of a piezoelectric element disposedin the actuator shown in FIG. 1.

FIG. 5 is a sectional view of the actuator along the line B-B in FIG. 1.

FIG. 6 is a block diagram of a control system.

FIG. 7 is a schematic view of an image forming device equipped with theactuator shown in FIG. 1.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention will be described with reference to theaccompanying drawings. The embodiments include an actuator, an opticalscanner and an image forming device.

FIG. 1 is a schematic plan view of an actuator according to a firstembodiment of the invention. FIG. 2 is a sectional view of the actuatoralong the line A-A in FIG. 1. FIG. 3 is a schematic sectional view fordescribing movements of a tension adjuster which is disposed in theactuator shown in FIG. 1. FIG. 4 is a schematic sectional view of apiezoelectric element disposed in the actuator shown in FIG. 1. FIG. 5is a sectional view of the actuator along the line B-B in FIG. 1. FIG. 6is a block diagram of a control system. In the following description,near, far, right and left sides in FIG. 1 will be referred to as“upper,” “lower,” “right,” and “left,” respectively. Upper, lower, rightand left sides in FIGS. 2, 3 and 5 will be referred to as “upper,”“lower,” “right,” and “left,” respectively for the purposes ofillustration.

Referring to FIG. 1, an actuator 1 includes a base body 2, a supportsubstrate 3 that supports the base body 2 through a spacer 4, a driver 5that rotates a movable plate 21 which is provided in the base body 2, atension adjuster 6 for adjusting the tension on a pair of axial parts221, 222 which is disposed in the base body 2, and a controller 7 thatcontrols the operation of the tension adjuster 6.

These components are now hereunder described. Referring to FIG. 1, thebase body 2 includes the movable plate 21, a pair of the axial parts221, 222 that support the movable plate 21 at the both sides of theplate, a pair of torsional axes 231, 232 that are coupled to the axialparts 221, 222 respectively, a support part 26 that supports thetorsional axes 231, 232 and has a frame shape, a pair of piezoelectricelement joint parts 251, 252 that are jointed with hereinafter-describedpiezoelectric elements 81, 82 respectively, and coupling parts 241, 242that couple the piezoelectric element joint parts 251, 252 with thetorsional axes 231, 232.

Referring to FIG. 1, in this embodiment, the pair of the axial parts221, 222, the pair of the torsional axes 231, 232, the pair of thepiezoelectric element joint parts 251, 252 and a pair of the couplingparts 241, 242 are disposed symmetrically with respect to the movableplate 21. The pair of the axial parts 221, 222, the pair of thetorsional axes 231, 232, the pair of the piezoelectric element jointparts 251, 252 and the pair of the coupling parts 241, 242 respectivelyhave the same structure each other in the pair.

The axial part 221, the torsional axis 231, the piezoelectric elementjoint part 251 and the coupling part 241 that are placed in the leftside with respect to the movable plate 21 in FIG. 1 are hereunderdescribed and the descriptions of the axial part 222, the torsional axis232, the piezoelectric element joint 252 and the coupling part 242 thatare disposed in the right side to the movable plate 21 in FIG. 1 will beomitted since those structures are the same as the corresponding onessituated in the left side.

The movable plate 21 has a disc shape in this embodiment. The shape ofthe movable plate 21 is not particularly limited to this. For example,it can be an oval shape, a square shape or the like when the plate isviewed in plan. A light reflecting part 221 which has a light reflectingproperty is formed on an upper face (a face opposite to the one closerto the support substrate 3) of the movable plate 21. A permanent magnet51 is provided on the lower face of the movable plate 21. The movableplate 21 is supported by the pair of the axial parts 221, 222 at itsboth sides.

Referring to FIG. 1, the axial part 221 has a rod shape. One end of theaxial part 221 in the longitudinal direction (the right side end inFIG. 1) is coupled to the movable plate 21 and the other end (the leftside end in FIG. 1) is coupled to the torsional axis 231. The pair ofthe axial parts 221, 222 are provided coaxially and the movable plate 21rotates with respect to the support part 26 around the axis or arotation central axis X.

Referring to FIG. 1, the torsional axis 231 is formed such that it istorsionally deformable around a Y axis which is placed orthogonal to therotation central axis X (or the axial direction of the axial parts 221,222) and to the thickness direction of the movable plate 21. Theactuator 1 adjusts the spring constant of the pair of the axial parts221, 222 by a hereinafter described drive source 61. The drive source 61deforms the torsional axis 231 torsionally around the Y axis and thetension on the pair of the axial parts 221, 222 is adjusted.

The torsional axis 231 has a long shape extending along the Y axis andits both ends are fixed to the support part 26. When the torsional axis231 is made so as to have the long shape extending along the Y axis, thetorsional axis 231 can be deformed torsionally around the Y axis with arelatively small force. The center part of the torsional axis 231 in thelongitudinal direction is coupled to the axial part 221. As describedabove, the torsional axis 231 is fixed to the support part 26 at itsboth ends in the longitudinal direction. Therefore the center part ofthe torsional axis 231 in its longitudinal direction is the part whichis most deformable around the Y axis (in other words the part which canbe torsionally deformed most largely with a smallest force). Because theaxial part 221 is jointed to such part according to the embodiment, itis possible to adjust the tension on the axial parts 221, 222 in anextensive area with a minimum force.

Moreover, where T1 denotes a thickness (thickness in the verticaldirection in FIG. 2) of the torsional axis 231 and T2 denotes athickness (thickness in the vertical direction in FIG. 2) of the axialpart 221, T1 is larger than T2 according to the embodiment. Furthermore,the Y axis is situated in a position lower than the joint part of thetorsional axis 231 and the axial part 221 as shown in FIG. 2. In thisway, the gap between the movable plate 21 and the support substrate 3can be made large and the movable plate 21 can be rotated with largeamplitude. The positional relation between the Y axis and the joint partof the torsional axis 231 and the axial part 221 is not particularlylimited. For example, the Y axis can be situated in upper side withrespect to the joint part of the torsional axis 231 and the axial part221 in FIG. 2.

The upper face of the torsional axis 231 and the upper face of the axialpart 221 are disposed so as to form a flat face. Thereby if the basebody 2 is formed by etching a silicon substrate for example, the axialpart 221 having the thickness T2 which is smaller than the thickness T1of the torsional axis 231 can be formed only by removing one side of thesilicon substrate. In this way it is possible to simplify themanufacturing process of the base body 2. However the upper face of thetorsional axis 231 and the upper face of the axial part 221 are notnecessarily disposed to form a flat face

Though it depends on the shape, size and the like of the torsional axis231, the degree of the torsion of the torsional axis 231 is usually verysmall when the tension on the pair of the axial parts 221, 222 isadjusted. Thereby the movable plate 21 is hardly displaced even when thetorsional axis 231 is torsionally deformed. In other words, the actuator1 can adjust the tension on the pair of the axial parts 221, 222 whilecontrolling the displacement of the movable plate 21. Therefore, wherethe actuator 1 is used as an optical device, the distance between anunshown light source and a light reflecting part 211 can be maintainedat a desired distance and a light beam which is emitted from the lightsource and reflected by the light reflecting part 211 can be directed toa desired position of a scanning object.

The piezoelectric element joint part 251 is disposed in a space 27 whichis defined by the torsional axis 231 and the left part of the supportpart 26. The piezoelectric element joint part 251 is placed an oppositeside to the movable plate 21 with respect to the torsional axis 231. Inthis way, freedom of the arrangement of the piezoelectric element jointpart 251 (disposition of the piezoelectric element 81) is increased andthe design freedom of the actuator 1 can be improved.

The piezoelectric element joint part 251 has a plate shape and thehereinafter described piezoelectric element 81 is jointed to the lowerface of the piezoelectric element joint part 251. Though the shape ofthe piezoelectric element joint part 251 is a substantially square inthis embodiment when it is viewed in plan from the movable plate 21, theshape is not limited to this and it can be a rectangular shape, a discshape or the like. The piezoelectric element joint part 251 is coupledto the torsional axis 231 through the coupling part 241.

The coupling part 241 has a long shape and is disposed along therotation central axis X when it is viewed in plan in FIG. 1. One end(the right end) of the coupling part 241 in its longitudinal directionis coupled to the torsional axis 231, and the other end (the left end)is coupled to the piezoelectric element joint part 251. The couplingpart 241 is jointed to the longitudinally center part of the torsionalaxis 231. A width of the coupling part 241 (the length in the Y-axisdirection) is slightly larger than the width of the axial part 221. Athickness of the coupling part 241 is about the same as the thickness ofthe torsional axis 231.

The shape of the coupling part 241 is not particularly limited providedthat it couples the piezoelectric element joint part 251 with thetorsional axis 231. For example, the coupling part 241 does notnecessarily have a long shape, the width of the coupling part 241 can besmaller than the width of axial part 221, and the thickness of thecoupling part 241 can be larger than the thickness of the torsional axis231. The coupling part 241, the piezoelectric element joint part 251 andthe piezoelectric element 81 form the drive source 61 which deforms thetorsional axis 231 torsionally around the Y axis. This means that thedrive source 61 is placed in the space 27.

The base body 2 is for example mainly made of silicon. The movable plate21, the pair of the axial parts 221, 222, the pair of the torsional axes231, 232, the pair of the piezoelectric element joint parts 251, 252,the pair of the coupling parts 241, 242 and the support part 26 areformed such that they form a single body. For example, a siliconsubstrate is provided and the silicon substrate is etched so as toobtain the planer shapes corresponding to the movable plate 21, the pairof the axial parts 221, 222, the pair of the torsional axes 231, 232,the pair of the piezoelectric element joint parts 251, 252, the pair ofthe coupling parts 241, 242 and the support part 26. In this way it ispossible to obtain the base body 2 in which the movable plate 21, thepair of the axial parts 221, 222, the pair of the torsional axes 231,232, the pair of the piezoelectric element joint parts 251, 252, thepair of the coupling parts 241, 242 and the support part 26 areintegrally formed. Moreover by adopting silicon as a main material, afine rotation characteristic can be realized and an excellent durabilitycan be obtained. Furthermore a micron-order processing(microfabrication) becomes possible thus the size of the actuator 1 canbe made small.

Referring to FIG. 2, the above-described base body is supported by thesupport substrate 3 through the spacer 4. The spacer 4 is for examplemainly made of glass or silicon. The spacer 4 is formed so as to havesubstantially the same shape as that of the support part 26 when it isviewed in plan. More specifically the spacer 4 has a frame shape.

An inner wall of the spacer 4 defines a space 41. The space 41 preventsthe movable plate 21 from contacting with the support substrate 3 whenthe actuator 1 is driving. A shape of the spacer 4 is not particularlylimited provided that it permits the rotation of the movable plate 21.The support substrate 3 has a plate shape and is mainly maid of forexample glass or silicon. The support substrate 3 is disposed so as tooppose the lower face (the face opposite to the one closer to the lightreflecting part 211) of the base body 2 with the spacer 4 interposedtherebetween.

Referring to FIG. 2, a coil 52 is provided on the upper face (the faceopposing the base body 2) of the support substrate 3 at a positionopposing to the movable plate 21. In other words, the coil 52 isdisposed so as to face the permanent magnet 51 which is disposed on thelower face of the movable plate 21. The coil 52 is coupled to anelectric current applicator 53. The permanent magnet 51, the coil 52 andthe electric current applicator 53 form the driver 5 that rotates themovable plate 21. The driver 5 will be described later in detail.

The piezoelectric element 81 is provided on the upper face of thesupport substrate 3 and at the position opposing to the piezoelectricelement joint part 251. In addition, the piezoelectric element 82 isprovided on the upper face of the support substrate 3 and at theposition opposing to the piezoelectric element joint part 252. Thepiezoelectric elements 81, 82 are now described but the piezoelectricelements 81, 82 have the same structure each other, therefore only thepiezoelectric element 81 is hereunder described and the description ofthe piezoelectric element 82 will be omitted.

The piezoelectric element 81 is disposed opposite to the lightreflecting part 211 with respect to the base body 2. Thereby it ispossible to increase the design freedom of the actuator 1. One end (anupper end) of the piezoelectric element 81 in its contraction andelongation direction is coupled to the lower face of the piezoelectricelement joint part 251, and the other end (a lower end) of thepiezoelectric element 81 is coupled to the upper face of the supportsubstrate 3. In other words, the piezoelectric element 81 is sandwichedbetween the support substrate 3 and the piezoelectric element joint part251. The piezoelectric element 81 elongates and contracts in thethickness direction of the movable plate 21 when electricity runsthrough the element.

Referring to FIG. 4, the piezoelectric element 81 has a square shapewhen it is viewed in plan. A plurality of piezoelectric layers 811having a piezoelectricity and a plurality of electrode layers 812 arealternately formed on top of each other and form the piezoelectricelement 811. A voltage is applied to the piezoelectric layer 811 throughthe adjacent electrode layer 812. With such layered-type piezoelectricelement 81, it is possible to lower the driving voltage and increase theamount of the displacement. The piezoelectric layers 811 are disposedsuch that the two adjacent piezoelectric layers 811 have an oppositepolarity direction. In other words, the piezoelectric layer 811 which issituated as an odd-numbered layer counted from the support substrate 3side among the plurality of the piezoelectric layers 811 has an oppositedirection of polarity to that of the piezoelectric layer 811 which issituated as an even-numbered layer. In this way, the driving voltage canbe securely reduced and the amount of the displacement of thepiezoelectric element 81 can be increased. In this description, the“polarity direction” means the direction in which a negative charge inthe plane of a piezoelectric layer where negative charges excessivelyexist moves to the plane of the piezoelectric layer where positivecharges excessively exist under the condition where neither an electricfield nor a stress is applied to the piezoelectric layer, the negativecharges excessively exit around one surface of the piezoelectric layerand the positive charges excessively exist around the other surface ofthe piezoelectric layer (spontaneous polarization or remanentpolarization).

Each electrode layer 812 is interposed between the two adjacentpiezoelectric layers 811. The plurality of the electrode layers 812 areprovided such that the two adjacent electrode layers 812 have anoverlapping area (an active region). Among the plurality of theelectrode layers 812, the odd-numbered electrode layers 812 counted fromthe support substrate 3 side are coupled to a common electrode 813 whichis disposed on a side face of the piezoelectric element 81, and theeven-numbered electrode layers 812 are coupled to a common electrode 814which is disposed on the other side face of the piezoelectric element81.

The common electrodes 813, 814 are coupled to a power supply circuit 83.The power supply circuit 83 applies a voltage to the common electrodes813, 814 and voltage is then supplied to each piezoelectric layer 811through the above-mentioned overlapping area. Accordingly eachpiezoelectric layer 811 is elongated and contracted in the thicknessdirection. The common electrodes 813, 814 are not necessarily disposedon the side faces of the piezoelectric element 81. They can be providedfor example on the support substrate 3. Moreover, the common electrodes813, 814 are not necessarily provided. The odd-numbered electrode layers812 counted from the support substrate side and the even-numberedelectrode layers 812 are respectively coupled directly with the powersupply circuit 83 instead.

As a piezoelectric material forming the piezoelectric layers 811, thereare for example zinc oxide, aluminum nitride, lithium tantalate, lithiumniobate, niobate potassium, piezoelectric zirconate titanate (PZT),barium titanate, and the like. One or more than one of theabove-mentioned material combined can be used to form the piezoelectriclayers 811. Particularly a material mainly composed of at least one ofthe zinc oxide, the aluminum nitride, the lithium tantalate, the lithiumniobate, the niobate potassium and piezoelectric zirconate titanate ispreferable. By adopting such material to form the piezoelectric layers811, it is possible to drive the actuator 1 at a higher frequency.

The actuator 1 according to the embodiment has been described. Suchactuator 1 torsionally deforms the torsional axis 231 through the actionof the drive source 61 and simultaneously deforms the torsional axis 232torsionally through the action of the drive source 62. In this way, theactuator 1 adjusts the tension on the pair of the axial parts 221, 222,and the spring constant of the axial parts 221, 222 can be adjusted. Inother words, the actuator 1 has the tension adjuster 6 that adjusts thespring constant of the axial parts 221, 222 with the pair of the axialparts 221, 222 and the pair of the drive sources 61, 62.

How the tension adjuster 6 works will be now described. The action ofthe drive source 61 is the same as that of the drive source 62 thereforeonly the action of the drive source 61 is described and the descriptionof the drive source 62 will be hereunder omitted.

Referring to FIG. 2, when the power supply circuit 83 is not inoperation (in other words no voltage is applied to the piezoelectricelement 81), the upper face of the piezoelectric element 81 is situatedat substantially the same level as the upper face of the movable plate21 in the thickness direction of the movable plate 21. In other words,the base body 2 is flat.

Referring to FIG. 3, when the piezoelectric element 81 is contracted bythe activated power supply circuit 83, the upper end of thepiezoelectric element 81 displaces downward together with thepiezoelectric element joint part 251. Thereby the left side of thecoupling part 241 is displaced downward in accordance with thedisplacement of the piezoelectric element joint part 251 and thecoupling part 241 leans. By leaning the coupling part 241, the torsionalaxis 231 is torsionally deformed around the Y axis.

The drive source 61 is provided such that it torsionally deforms thetorsional axis 231 toward the direction in which the tension of the pairof the axial parts 221, 222 is increased. In other words, the drivesource 61 deforms the torsional axis 231 torsionally such that the jointpart of the torsional axis 231 with the axial part 221 is displacedtoward the direction in which the distance between the joint part andthe movable plate 21 is increased. More specifically, the drive source61 deforms the torsional axis 231 torsionally in a counter-clock wisedirection in FIG. 2.

The spring constant of the pair of the axial parts 221, 222 can beadjusted when the drive source 61 deforms the axial part 221 torsionallytoward the direction in which the distance between the joint part of thetorsional axis 231 with the axial part 221 and the movable plate 21 isdecreased. However, in this case, the pair of the axial parts 221, 222can be warped and the movable plate 21 can be displaced in the thicknessdirection though it depends on the degree of the torsion of thetorsional axis 231.

Whereas the torsional axis 231 is deformed such that the distancebetween the joint part of the torsional axis 231 with the axial part 221and the movable plate 21 is increased, the warp of the axial parts 221,222 is prevented. Therefore the actuator 1 can adjust the springconstant of the axial parts 221, 222 while securing the stable rotationof the movable plate 21. As described above, the coupling part 241 isjointed to the longitudinally center part of the torsional axis 231. Thecenter part of the torsional axis 231 in its longitudinal direction isthe part which is most deformable so that it is possible to deform thetorsional axis 231 torsionally around the Y axis with a smallest forceby jointing the coupling part 241 with the such part.

Moreover, since the coupling part 241 is disposed along the rotationcentral axis X, the force generated through the deformation of thepiezoelectric element 81 can be efficiently transmitted to the torsionalaxis 231. Furthermore, the Y axis is situated in a position lower thanthe joint part of the torsional axis 231 with the axial part 221. Inthis way, it is possible to dispose the piezoelectric element 81 betweenthe piezoelectric element joint part 251 and the support substrate 3.Accordingly the torsional axis 231 can be torsionally deformed towardthe direction in which the tension on the axial parts 221, 222 isincreased by contracting the piezoelectric element 81.

Consequently it is possible to downsize the actuator 1 because thepiezoelectric element 81 is disposed in the space 41. Moreover thespring constant of the pair of the axial parts 221, 222 is adjusted bycontracting the piezoelectric element 81 so that it is possible toprevent the piezoelectric element joint part 251 from protruding out ofthe upper face of the base body 2. In this aspect, the size of theactuator 1 can be reduced. With the drive source 61 according to theembodiment, the torsional axis 231 can be torsionally deformed by arelatively simple structure. Furthermore with such structure, a largetorque for deforming the torsional axis 231 torsionally can be generatedand it becomes easy to maintain the torsion of the torsional axis 231 ata desired degree.

The tension adjuster 6 has the pair of the torsional axes 231, 232 andthe pair of the drive sources 61, 62 so that the spring constant of thepair of the axial parts 221, 222 can be adjusted in a wide range. Byusing such tension adjuster 6, it is possible to adjust the springconstant of the pair of the axial parts 221, 222 even after the actuatoris completed, and possible to provide the actuator 1 which can exercisea desired vibration characteristic. Moreover the spring constant of thepair of the axial parts 221, 222 can be adjusted even when the actuator1 is driving thereby it is possible for the actuator 1 to exert anexcellent rotational characteristic.

The driver 5 that rotates the movable plate 21 is now described. Asdescribed above, the permanent magnet 51, the coil 52 and the electriccurrent applicator 53. The permanent magnet 51 is disposed along theplate face of the movable plate 21. Referring to FIG. 1 and FIG. 5, thepermanent magnet 51 has a long shape and extends along the directionorthogonal to the rotation central axis X. The permanent magnate 51 ismagnetized in its, longitudinal direction. More specifically, one end ofthe permanent magnet 51 is a north pole and the other end is a southpole with respect to the rotation central axis X.

Any magnet can be used as the permanent magnet 51, for example,neodymium magnet, ferrite magnet, samarium-cobalt magnet, and alnicomagnet can be preferably used. The coil 52 is coupled to the electriccurrent applicator 53. When an alternating current is applied to thecoil 52 by the electric current applicator 53, a magnetic filed having amagnetic field line which extends around the coil 52 in a directionperpendicular to the plate face of the movable plate 21 is generated andthe direction of the magnetic field is alternatively switched.

More specifically, two states alternate: one state (a first state) isthat the right side of the permanent magnet 51 with respect to therotation central axis X moves toward the coil 52 and the left part movesaway from the coil 52; the other state (a second state) is that theright side of the permanent magnet 51 with respect to the rotationcentral axis X moves away from the coil 52 and the left part movestoward the coil 52.

The movable plate 21 rotates around the rotation central axis X as thefirst state and the second state are alternatively repeated. Though analternating current is applied to the coil 52 by the electric currentapplicator 53 in the above-described embodiment, it is not limited tothis as long as the movable plate 21 is rotated. For example, theelectric current applicator 53 can apply direct current to the coil 52intermittently.

When the actuator 1 is actuated, it is preferable that the frequency ofthe current (hereinafter also referred as to “driving frequency V1”)applied to the coil 52 by the electric current applicator 53 beidentical to the resonance frequency of the movable plate 21(hereinafter also referred as to “resonance frequency V2”). In this way,it is possible to rotate the movable plate 21 with large amplitude aswell as lowering the driving voltage level. In this description, the“resonance frequency of the movable plate 21” means a torsionalresonance frequency of a vibration system including the movable plate 21and the pair of the axial parts 221, 222. The resonance frequency of themovable plate 21 depends on the weight of the movable plate 21(including weights of the light reflecting part 211 and the magnet 51)and the spring constant (stiffness in torsion) of the pair of the axialparts 221, 222.

The actuator 1 is set to adjust the tension on the pair of the axialparts 221, 222 (or the spring constant of the pair of the axial parts221, 222) through the tension adjuster 6 such that the resonancefrequency V2 becomes equal to the driving frequency V1. The drivingfrequency V1 is generally changed according to an application of theactuator 1. Therefore the resonance frequency V2 is adjusted to thedriving frequency V2 and the actuator 1 which can exercise a desiredvibration characteristic can be achieved.

According to the embodiment, the torsional axis 231 is torsionallydeformed toward the direction in which the tension on the pair of theaxial parts 221, 222 is increased or the spring constant of the pair ofthe axial parts 221, 222 is made higher. Thus it is preferable that theactuator 1 be designed such that the resonance frequency V2 is slightlylower than the driving frequency V1 at the time when the actuator 1 ismanufactured.

How to adjust the resonance frequency V2 to the driving frequency V1 isnow described. The actuator 1 has a resonance frequency detector 9 thatdetects the resonance frequency V2, and a controller 7 that controls theoperation of the tension adjuster 6 based on the detection result of thetension adjuster 6. For example, the resonance frequency detector 9detects the resonance frequency V2 right after the power activation ofthe actuator 1 is performed, the controller 7 then controls theoperation of the power supply circuit 83 based on the detected resonancefrequency V2. The detection of the resonance frequency V2 by theresonance frequency detector 9 is not necessarily performed right afterthe power activation, the detection of the resonance frequency V2 can befor example conducted at regular time intervals after the poweractivation.

The resonance frequency detector 9 is disposed on the axial part 221.The resonance frequency detector 9 includes a detection element 91 whoseresistance value is changed in accordance with its deformation, and adetection part 92. The detection part 92 detects the behavior (orvibration amplitude) of the movable plate 21 based on the resistancevalue change of the detection element 91 and determines the resonancefrequency V2 from the detected behavior of the movable plate 21. Sincethe axial part 221 torsionally deforms around the rotation central axisX when the movable plate 21 rotates, the detection element 91 that isdisposed on the axial part 221 is deformed according to the torsionaldeformation of the axial part 221. Thereby the resistance value of thedetection element 91 changes.

The detection part 92 has for example an unshown power source unit whichapplies an voltage to the detection element 91 and an unshown electriccurrent measurement part that detects an electric current change (or theresistance value change of the detection element 91) in the circuitincluding the power source unit and the detection element 91. Thebehavior of the movable plate 21 is detected based on the change in theelectric current value detected by the electric current measurementpart, and the resonance frequency V2 is determined based on thedetection result.

The controller 7 for example increases the voltage which is applied tothe piezoelectric element 81 and supplied from the power supply circuit83 (the same thing can be applied to the voltage supplied from the powersupply circuit 84 and applied to the piezoelectric element 82) till theresonance frequency V2 becomes same as the driving frequency V1. Inother words, the controller 7 gradually contracts the piezoelectricelement 81 and then controls the power supply circuit 83 such that thevoltage which is supplied from the power supply circuit 83 and appliedto the piezoelectric element 81 is maintained at a value where theresonance frequency V2 becomes identical to the driving frequency V1. Inthis way, the actuator 1 adjusts the resonance frequency V2 to thedriving frequency V1. The controller 7 controls the operation of thepower supply circuit 83 when it is necessary. Therefore if a givendriving frequency V1 corresponds with a resonance frequency V2 which isdetected by the resonance frequency detector 9, the power supply circuit83 does not have to be operated.

The controller 7 does not necessarily control the power supply circuit83 such that its voltage is gradually increased. For example, severalvoltage values corresponding to margins between the resonance frequencyV2 and the driving frequency V2 can be given in advance, and thecontroller 7 can control the power supply circuit 83 such that itapplies a selected voltage which is selected out of the given voltagevalues based on the detection result of the resonance frequency detector9.

The actuator according to the embodiment has been described. Theactuator has the light-reflecting part thereby the actuator 1 can beapplied to various optical devices such as an optical scanner, anoptical switch and an optical attenuator.

An optical scanner according to an embodiment of the invention has thesame structure as the above-described actuator except that alight-reflecting part having a light-reflection property is provided onone face of the movable plate. Therefore the embodiment of the opticalscanner will not be described in detail here. The optical scanneraccording to the embodiment can be applied to various image formingdevice such as a projector, a laser printer, a display for imaging, abar-code reader and a confocal scanning microscope. Consequently it ispossible to provide an image forming device that has a fine imagedrawing characteristic.

A projector 100 using the actuator 1, which is a specific example of theimage forming device, is now described with reference to FIG. 7. Alongitudinal direction of a screen S is refereed as a “transversedirection” and an orthogonal direction to the longitudinal direction isreferred as a “vertical direction” for convenience of explanation. Theprojector 100 has a light-source unit 101 that emits light such as alaser beam, a plurality of dichroic mirrors 102 and a pair of theactuators 1, 1.

The light-source unit 101 includes a red light source device 101 a thatemits a red light beam, a blue light source device 101 b that emits ablue light beam, and a green light source device 101 c that emits agreen light beam. The dichroic mirror is composed of four rectangularprisms which are bonded together and is an optical element whichcombines the light beams emitted from the red light source device 101 a,the blue light source device 101 b and the green light source device 101c. The combined light beam is scanned by the actuators 1, 1 and it formsa color image on the screen S.

Optical scanning by the actuators 1, 1 is described in detail. The lightbeam combined at the dichroic mirrors 102 is scanned (main scanning) inthe transverse direction of the screen S by one of the actuators 1, 1.The transversely scanned light beam is then scanned (vertical scanning)in the vertical direction of the screen S by the other actuator 1.Through such process, a two-dimension colored image can be formed on thescreen S.

Though the actuator, the optical scanner and the image forming deviceaccording to the embodiment have been described with reference to theaccompanying drawings, the invention is obviously not limited to thespecific embodiments described herein, but also encompasses anyvariations that may be considered by any person skilled in the art,within the general scope of the invention. The invention concerning theactuator, the optical scanner and the image forming device alsoencompasses the constructions that serve the equivalent function andexert the equivalent effect as those of the embodiments. The inventionalso encompasses the structure in which a hitherto know art is added tothe structure described in the above embodiments.

Though the actuator has the symmetrical structure with respect to themovable plate in the above-described embodiment, the actuator 1 may havea asymmetrical structure. Moreover, through the tension adjuster has thepair of the torsional axes which is provided corresponding to the pairof the axial parts, and the pair of the drive sources which is providedcorresponding to the pair of the torsional axes, the structure is notlimited to this provided that it has at least a pair of the torsionalaxis and the drive source. Where the actuator has only a pair of thetorsional axis and the drive source, an end of the axial part that isnot coupled with the torsional axis in the two axial parts is fixed tothe frame-shaped support part.

Moreover, the piezoelectric element has the layered structure in theembodiment. However the element can have a single-layer structure aslong as the piezoelectric element joint part can be displaced. Thoughthe torsional axis is torsionally deformed when the piezoelectricelement is contracted in the above-described embodiment, the torsionalaxis can be torsionally deformed when the piezoelectric element iselongated. Furthermore, so-called electromagnetic driving method usingthe permanent magnet and the coil is adopted as the driving means whichrotates the movable plate in the embodiment. However other methods canbe adopted to drive the movable plate. For example, a so-calledpiezoelectric driving method in which the axial part is torsionallydeformed through contraction and elongation of a piezoelectric elementand the movable plate is rotated, a so-called electrostatic drivingmethod in which the movable plate is rotated through a coulomb forcegenerated between the movable plate and the support substrate, or thelike can be adopted.

1. An actuator, comprising: a movable plate having a plate shape; a pairof axial parts that is elastically deformable and supporting the movableplate rotatable; and a tension adjuster adjusting tension on an axialdirection of the pair of the axial parts and including a torsional axisthat is formed jointly or integrally with one of the axial parts anddisposed orthogonally to the axial direction of the axial parts, and adrive source that torsionally deforms the torsional axis, wherein thetorsional axis is torsionally deformed through operation of the drivesource and a spring constant of the pair of the axial parts is adjustedby adjusting the tension on the pair of the axial parts.
 2. The actuatoraccording to claim 1, the tension adjuster having a pair of thetorsional axes corresponding to the pair of the axial parts and a pairof the drive sources corresponding to the pair of the torsional axes. 3.The actuator according to claim 1, wherein a torsional center of thetorsional axis is disposed at a distance from a joint part of thetorsional axis with the axial part in a thickness direction of themovable plate.
 4. The actuator according to claim 1, wherein both endsof the torsional axis in its axial direction are fixed.
 5. The actuatoraccording to claim 4, wherein the torsional axis is coupled with theaxial part at a center part of the torsional axis in the axialdirection.
 6. The actuator according to claim 1, wherein the drivesource torsionally deforms the torsional axis toward a direction inwhich the tension on the pair of the axial parts is increased.
 7. Theactuator according to claim 1, the drive source further comprising apiezoelectric element contracting and elongating in a directionorthogonal to the axial direction of the axial part and to an axialdirection of the torsional axis, a piezoelectric element joint partjointed with an end of the piezoelectric element in a contraction andelongation direction of the piezoelectric element, and a coupling partcoupling the piezoelectric element joint part with the torsional axis,wherein the piezoelectric element joint part is displaced throughcontraction and elongation of the piezoelectric element, and thetorsional axis is torsionally deformed through the displacement of thecoupling part caused by the contraction and elongation of thepiezoelectric element.
 8. The actuator according to claim 7, wherein thepiezoelectric joint part is disposed in an opposite side to the movableplate with respect to the torsional axis.
 9. The actuator according toclaim 8, wherein the coupling part is coupled with a center part of thetorsional axis in the axial direction of the torsional axis.
 10. Theactuator according to claim 9, wherein the coupling part is disposedalong an axial line of the axial part when it is viewed from a plane ofthe movable plate.
 11. The actuator according to claim 7, wherein thepiezoelectric element has a structure in which piezoelectric layers andelectrode layers are alternately deposited on top of each other.
 12. Theactuator according to claim 1, further comprising: a driver rotating themovable plate where electricity is turned on, wherein the tensionadjuster adjusts the tension on the pair of the axial parts such that afrequency of a voltage that is applied to the driver becomes same as aresonance frequency of the movable plate.
 13. The actuator according toclaim 1, further comprising: a light reflecting part having a lightreflection property and disposed on one face of the movable plate. 14.The actuator according to claim 13, the drive source further comprisinga piezoelectric element contracting and elongating in a directionorthogonal to the axial direction of the axial part and to an axialdirection of the torsional axis, a piezoelectric element joint partjointed with an end of the piezoelectric element in a contraction andelongation direction of the piezoelectric element, and a coupling partcoupling the piezoelectric element joint part with the torsional axis,wherein the piezoelectric element is disposed in an opposite side to thelight reflecting part with respect to the movable plate.
 15. An opticalscanner, comprising: a movable plate having a plate shape and having alight reflecting part that has a light reflecting property and isdisposed on a plate face of the movable plate; a pair of axial partsthat is elastically deformable and supporting the movable platerotatable; and a tension adjuster adjusting tension on an axialdirection of the pair of the axial parts and including a torsional axisthat is formed jointly or integrally with one of the axial parts anddisposed orthogonally to the axial direction of the axial parts, and adrive source that torsionally deforms the torsional axis, wherein thetorsional axis is torsionally deformed through operation of the drivesource and a spring constant of the pair of the axial parts is adjustedby adjusting the tension on the pair of the axial parts.
 16. An imageforming device comprising: an optical scanner including: a movable platehaving a plate shape and having a light reflecting part that has a lightreflecting property and is disposed on a plate face of the movableplate; a pair of axial parts that is elastically deformable andsupporting the movable plate rotatable; and a tension adjuster adjustingtension on an axial direction of the pair of the axial parts andincluding a torsional axis that is formed jointly or integrally with oneof the axial parts and disposed orthogonally to the axial direction ofthe axial parts, and a drive source that torsionally deforms thetorsional axis, wherein the torsional axis is torsionally deformedthrough the action of the drive source and a spring constant of the pairof the axial parts is adjusted by adjusting the tension on the pair ofthe axial parts.